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J/A+A/501/563 HKs photometry in the Arches cluster (Espinoza+, 2009)
The massive star initial mass function of the Arches cluster.
Espinoza P., Selman F.J., Melnick J.
<Astron. Astrophys. 501, 563 (2009)>
=2009A&A...501..563EADC_Keywords: Clusters, open ; Photometry, infrared ; Stars, masses
Keywords: Galaxy: open clusters and associations: individual: Arches -
stars: early-type - instrumentation: adaptive optics -
ISM: dust, extinction - stars: luminosity function, mass function
Abstract:
The massive Arches cluster near the Galactic center should be an ideal
laboratory for investigating massive star formation under extreme
conditions. But it comes at a high price: the cluster is hidden behind
several tens of magnitudes of visual extinction. Severe crowding
requires space or AO-assisted instruments to resolve the stellar
populations, and even with the best instruments interpreting the data
is far from direct. Several investigations using NICMOS and the most
advanced AO imagers on the ground revealed an overall top-heavy IMF
for the cluster, with a very flat IMF near the center. There are
several effects, however, that could potentially bias these results,
in particular the strong differential extinction and the problem of
transforming the observations into a standard photometric system in
the presence of strong reddening. We present new observations obtained
with the NAOS-Conica (NACO) AO-imager on the VLT. The problem of
photometric transformation is avoided by working in the natural
photometric system of NACO, and we use a Bayesian approach to
determine masses and reddenings from the broad-band IR colors. A
global value of Gamma=-1.1±0.2 for the high-mass end (M>10M☉)
of the IMF is obtained, and we conclude that a power law of Salpeter
slope cannot be discarded for the Arches cluster. The flattening of
the IMF towards the center is confirmed, but is less severe than
previously thought. We find Gamma=-0.88±0.20, which is incompatible
with previous determinations. Within 0.4pc we derive a total mass of
∼2.0(±0.6)x104M☉ for the cluster and a central mass density
rho=2(±0.4)x105M☉/pc3 that confirms Arches as the densest
known young massive cluster in the Milky Way.
Description:
The NAOS-CONICA data (ESO Program ID 073.D-0815) were obtained under
clear weather conditions with subarcsecond seeing. The detector was an
Aladdin 1024x1024 pixel InSb array and the camera had a plate scale of
27.15[mas/pix], giving us a 27x27arcsec2 field of view of the
Arches cluster. Total integration times were 1000, 400 and 720[s] in
J, H, and Ks respectively, with the telescope moving alternatively to
sky positions for a proper background subtraction.
To optimize the Adaptive Optics (AO) performance we used the N90C10
dichroic, i.e. 90% of the light was directed to the infrared wavefront
sensor. The Strehl ratio of our observations exceeded 27% in Ks, and
reached more modest values of 5% in J, and 11% in the H band.
Tables 2 and 3 present the DAOPHOT photometry of 427 HKS and 126 JHKS
stars in the innermost 10 arcseconds of the Arches cluster. Table 3 is
considerably shorter due to the increasing extinction towards bluer
wavelengths. Table 5 presents the catalog with all the observed data
and physical parameters derived from the Bayesian method and using the
Color-magnitude stereogram.
File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file
table2.dat 57 427 HKs photometry of stars in the Arches cluster
table3.dat 57 126 JHKs photometry of stars in the Arches cluster
table5.dat 68 126 Physical parameters of JHKs stars
See also:
J/ApJ/581/258 : Infrared photometry in the Arches Cluster (Figer+, 2002)
Byte-by-byte Description of file: table[23].dat
Bytes Format Units Label Explanations
1- 5 I5 --- Seq Star unique ID in this work (G1)
7- 12 F6.3 arcsec dRA Right Ascension offset (2)
14- 19 F6.3 arcsec dDE Declination offset (2)
21- 26 F6.3 mag Ksmag NACO Ks band magnitude
28- 32 F5.3 mag e_Ksmag 1σ uncertainty in Ks
34- 39 F6.3 mag H-Ks NACO (H-Ks) color index
41- 45 F5.3 mag e_H-Ks 1σ uncertainty in (H-Ks)
47- 51 F5.3 mag J-H ? NACO (J-H) color index (table3 only)
53- 57 F5.3 mag e_J-H ? 1σ uncertainty in (J-H) (table3 only)
Note (2): Positions with respect to 17:45:50.798-28:49:25.606 (J2000).
Byte-by-byte Description of file: table5.dat
Bytes Format Units Label Explanations
1- 5 I5 --- Seq Star unique ID in this work (G1)
7- 12 F6.3 mag Ksmag NACO Ks band magnitude
13- 18 F6.3 mag H-Ks NACO (H-Ks) color index
20- 24 F5.3 mag J-H NACO (J-H) color index
26- 31 F6.3 mag VMAG Absolute V band magnitude (3)
33- 37 F5.3 [solLum] logL Log of the luminosity (3)
39- 43 F5.3 [K] logTeff Log of the effective temperature (3)
45- 49 F5.3 [N/kg] logg Log of the surface gravity (3)
51- 55 F5.3 mag E(H-Ks) Color excess
57- 62 F6.2 solMass Mini Initial mass (3)
64- 68 F5.2 solMass Mact Present day mass (3)Note (3): Assuming that the stars in the Arches cluster formed 2.5Myr ago,
have solar metallicity, and are located at a distance modulus of 14.52
magnitudes. We compared our observations against the Geneva models
with standard mass-loss rates.
Global notes:
Note (G1): The star ID is designated by sorting the data in order of
increasing Ks magnitude. Only stars inside a radius of 10 arcseconds
from the cluster center are included in this catalog.
Acknowledgements:
Pablo Espinoza, pespinoza(at)as.arizona.edu
(End) Pablo Espinoza [Univ. of Arizona, USA], Patricia Vannier [CDS] 02-Jun-2009